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designing process
A. Create a netlist
1. The network table is the interface file between the schematic diagram and the PCB board. The PCB designer should select the correct network table format according to the schematic diagram and the characteristics of the PCB design tool used, and create a network table that meets the requirements.
2. In the process of creating the network table, according to the characteristics of the schematic design tool, actively assist the schematic designer to eliminate errors. Ensure the correctness and completeness of the netlist.
3. Determine the package of the device (PCB FOOTPRINT).
4. Create PCB board Create PCB design file according to single board structure drawing or corresponding standard board frame;
Pay attention to the correct selection of the position of the coordinate origin of the veneer, the principle of origin setting:
A. The intersection of the left and lower extension lines of the veneer.
B. The first pad in the lower left corner of the board.
Rounded corners around the frame, with a chamfering radius of 5mm. Refer to the structural design requirements for special circumstances.
B. Layout
1. Set the size of the board frame according to the structure drawing, arrange the mounting holes, connectors and other devices that need to be positioned according to the structural elements, and give these devices immovable attributes. Carry out dimension marking according to the requirements of process design specifications.
2. Set the forbidden wiring area and layout area of the printed board according to the structure drawing and the clamping edge required during production and processing. According to the special requirements of certain components, a wiring prohibition area is set.
3. Comprehensive consideration of PCB performance and processing efficiency to select the processing flow.
The preferred sequence of the processing technology is: single-sided mounting on component side-component side mounting, insert and mixed mounting (component side mounting and soldering surface mounting once wave forming)-double-sided mounting-component side mounting and insert mixed mounting, Soldering surface mount.
4. Basic principles of layout operation
A. Follow the layout principle of "large first, then small, difficult first, easy first", that is, important unit circuits and core components should be laid out first.
B. The principle block diagram should be referred to in the layout, and the main components should be arranged according to the main signal flow law of the single board.
C. The layout should meet the following requirements as far as possible: the total wiring is as short as possible, and the key signal line is the shortest; high voltage, large current signal and low current, low voltage weak signal are completely separated; analog signal and digital signal are separated; high frequency signal Separate from low-frequency signals; the spacing of high-frequency components should be sufficient.
D. For the circuit parts of the same structure, adopt the "symmetrical" standard layout as much as possible;
E. Optimize the layout according to the standards of uniform distribution, balanced center of gravity, and beautiful layout;
F. Device layout grid setting. In general IC device layout, the grid should be 50-100 mil. For small surface mount devices, such as surface mount component layout, the grid setting should be no less than 25mil.
G. If there are special layout requirements, they should be determined after communication between the two parties.
5. The same type of plug-in components should be placed in one direction in the X or Y direction. The same type of polarized discrete components should also strive to be consistent in the X or Y direction to facilitate production and inspection.
6. The heating elements should generally be evenly distributed to facilitate the heat dissipation of the veneer and the whole machine. Temperature sensitive devices other than the temperature detection element should be kept away from the components that generate large amounts of heat.
7. The arrangement of components should be convenient for debugging and maintenance, that is, large components cannot be placed around small components, components that need to be debugged, and there must be enough space around the components.
8. For veneers produced by wave soldering, the fastener installation holes and positioning holes should be non-metallized holes. When the mounting hole needs to be grounded, it should be connected to the ground plane by means of distributed grounding holes.
9. When the wave soldering production process is used for the mounting components of the soldering surface, the axis of the resistor and capacitor should be perpendicular to the wave soldering transmission direction, and the axis of the resistance row and SOP (PIN distance greater than or equal to 1.27mm) components should be parallel to the transmission direction; Avoid wave soldering for active components such as IC, SOJ, PLCC, QFP, etc. whose PIN pitch is less than 1.27mm (50mil).
10. The distance between BGA and adjacent components>5mm. The distance between other SMD components>0.7mm; the distance between the outside of the mounting component pad and the outside of the adjacent interposing component is greater than 2mm; PCB with crimping parts, no interposing within 5mm of the crimped connector Components and devices shall not be mounted within 5mm of the welding surface.
11. The layout of the IC decoupling capacitor should be as close as possible to the power supply pin of the IC, and the loop between it and the power supply and ground should be the shortest.
12. In the component layout, appropriate consideration should be given to putting the devices using the same power supply together as much as possible to facilitate future power supply separation.
13. The layout of the resistance vessel used for impedance matching purposes should be arranged reasonably according to its properties.
The layout of the series matching resistor should be close to the driving end of the signal, and the distance generally does not exceed 500 mil.
The layout of matching resistors and capacitors must distinguish the source and terminal of the signal. For multi-load terminal matching, it must be matched at the farthest end of the signal.
14. After the layout is completed, print out the assembly drawing for the schematic designer to check the correctness of the device package, and confirm the signal correspondence between the single board, the backplane and the connector, and then start wiring after confirming that it is correct.
